Lead sulfide (galena) is usually found in association with iron sulfide (pyrite), and often with zinc sulfide (sphalerite). The conventional procedure for treating ores of such composition is by selective froth flotation, to obtain lead in the form of a high grade lead concentrate and zinc in the form of a high grade zinc concentrate. Such concentrates then are treated separately for the recovery of their metal values.
However, many fine-grained massive sulfide lead-zinc ores cannot be satisfactorily treated by flotation to produce a high grade lead concentrate. If the lead cannot be liberated satisfactorily by grinding, dilution of the concentrate by pyrite can result in a concentrate which is not suited for treatment by conventional methods, or it may result in inability to produce separate lead and zinc concentrates adequately freed of zinc and lead respectively. Efforts to produce satisfactory concentrates from such ores by refinements in flotation techniques have been the chief procedure used heretofore to treat such ores, but have met with only indifferent success. Smelting processes heretofore proposed to treat mixed lead-zinc concentrates, and low grade lead concentrates high in iron sulfide, are costly and unsatisfactory.
It has been known heretofore that zinc from zinc sulfide ores and concentrates will dissolve in an ammoniacal solution under oxidizing conditions. For example, the Forward U.S. Pat. No. 2,822,263, which is directed primarily to the recovery of copper and nickel from sulfide ores and concentrates by leaching under pressure with an oxidizing ammoniacal solution, mentions that zinc also dissolves in such leaching operations. However, lead sulfide generally does not dissolve in ammoniacal solutions, and no proposal has to our knowledge been made heretofore to use such solutions for recovery or upgrading the lead content of lead sulfide concentrates.